Recently, infrared radiation elements manufactured by use of manufacturing techniques or the like for MEMS (micro electro mechanical systems) have been studied and developed. These kinds of infrared radiation elements may be used as infrared sources for gas sensors, optical analysis devices, or the like.
A radiation source which has a configuration shown in FIGS. 8 and 9 and is disclosed by Patent document 1 (JP H09-184757 A) is known as one of these kinds of infrared radiation elements.
This radiation source includes: a substrate 13; a first insulating layer 22 on the substrate 13; a radiation surface layer 11 on the first insulating layer 22; a second insulating layer 24 on the radiation surface layer 11; and two or more extremely narrow incandescent filaments 10 on the second insulating layer 24. Besides, the radiation source further includes: a third insulating layer 26 to cover and protect the incandescent filaments 10; and a pair of pads 15 and 15 which are connected respectively to both ends of each incandescent filament 10 through openings of the third insulating layer 26. The second insulating layer 24 is provided for insulating the radiation surface layer 11 from the incandescent filaments 10. Patent document 1 discloses that the incandescent filaments 10 are enclosed by other components (first insulating layer 22, radiation surface layer 11, second insulating layer 24, and third insulating layer 26) which compose a uniform plane plate having a multilayer structure. Besides, Patent document 1 discloses that the purpose of providing the first insulating layer 22 and the third insulating layer 26 is to protect the incandescent filaments 10 and the radiation surface layer 11 from oxidation.
The substrate 13 has an opening 14 corresponding to the radiation surface layer 11. Patent document 1 discloses that an etchant to form the opening 14 may be an aqueous potassium hydroxide (KOH) solution, an aqueous ethylenediamine solution containing a small amount of pyrocatechol, or tetramethyl ammonium hydroxide (TMAH).
The substrate 13 is made of a (100)-oriented silicon chip. The first insulating layer 22 is a silicon nitride layer with the thickness of 200 nm. The radiation surface layer 11 is a polysilicon film which is doped with boron, phosphorus, or arsenic, and has the thickness of about 1 μm. The second insulating layer 24 is a nitride silicon layer with the thickness of about 50 nm. Each incandescent filament 10 is a tungsten layer with the thickness of about 400 nm. The third insulating layer 26 is a silicon nitride layer with the thickness of about 200 nm. The metal pad 15 is made of aluminum, for example and in ohmic contact with the incandescent filaments 10 through the opening of the third insulating layer 26.
In the radiation source, the radiation surface layer 11 has an area of 1 mm2. The incandescent filaments 10 each have a size that the thickness thereof ranges from 0.1 to 1 μm, the width thereof ranges from 2 to 10 μm, and an interval therebetween ranges from 20 to 50 μm, for example.
In the radiation source, when currents flow through the incandescent filaments 10, the temperatures of the incandescent filaments 10 increase. In this regard, the incandescent filaments 10 are mainly used for heating the radiation surface layer 11, and the radiation surface layer 11 serves as a main heat radiation source.
By the way, it is known that, when an infrared radiation element is used as an infrared ray source for a spectroscopic gas sensor, for example, an S/N ratio of an output of the gas sensor is improved by intermittently driving the infrared radiation element to intermittently radiate infrared radiation and amplifying an output of a light receiving element with a lock-in amplifier.
However, when the configuration shown in FIGS. 8 and 9 is adopted for the radiation source, a response in temperature change of the radiation surface layer 11 to a voltage waveform applied to the incandescent filaments 10 is delayed due to heat capacities of the first insulating layer 22, the radiation surface layer 11, the second insulating layer 24, and the third insulating layer 26 in addition to a heat capacity of the incandescent filament 10. Therefore, in the above-mentioned radiation source, the temperature of the radiation surface layer 11 is less likely to increase, and it is difficult to lower energy consumption and increase a rate of the response.